PO Release with reason for Rejection comments with pop up

Hi,

my reqiurement is to  get PO release when PO changed .

1. i copied standard workflow WS20000075.

2. when user reject release of PO then we need to dispaly popup screen to get  rejected reason comments .

how to get that pop up screen ? will it automatically dispaly popup scren when user reject PO release in ME29 ? .

   or

do we need to do any development ?

from previous forms i came to know we can use 1. CATSXT_SIMPLE_TEXT_EDITOR

                                                                                    or

                                                                      2. POPUP_GET_VALUES

but i need to display popup when user click reject button of PO release .is it possible using  function modules.

please tell where we have to use these function module and how to use that function module.

please help me..